Oxyfiring or oxy-fuel combustion of fuel in heaters and boilers is a promising carbon dioxide capture process in which fuel is burned generally in the presence of high-purity oxygen, instead of air, to produce heat and a flue gas. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide and trace amounts of other gases. Normal air, as compared to dry air, contains a variable amount of water vapor, on average around 1%. The oxyfiring process avoids the need to separate nitrogen gases and other trace gases from the flue gas, which would otherwise be present if air rather than primarily oxygen were burned. Furthermore, the formation of nitrous oxide is avoided or at least substantially reduced with oxyfiring.
The flue gas is composed primarily of carbon dioxide and water vapor with other trace pollutants dependent on the fuel selected. The flue gas can be readily cooled separating out the water and producing a concentrated carbon dioxide stream, which can be further purified to obtain a high-purity carbon dioxide stream. This stream can be sequestered to prevent the release of carbon dioxide into the atmosphere. For example, the carbon dioxide stream may be pumped into a subterranean reservoir. Alternatively, the carbon dioxide may be captured and compressed to a liquid, which can then be sold to industry. Otherwise, the carbon dioxide can be reacted with other chemicals to produce a solid.
Conventionally, one or more cryogenic air separation units (ASU) supplies oxygen for the oxyfiring process. However, the use of ASU systems can be expensive in terms of both upfront capital expenses and ongoing operating expenses.
Another means of providing oxygen for oxyfiring is through the use of ion transport membranes (ITMs). ITMs are generally permeable to oxygen and no other substance and require significantly less energy per unit of oxygen than most conventional methods for producing oxygen. BOC has developed a process to produce an oxygen stream for oxyfuel combustion process using mixed ionic and electronic conductor materials.
Mattisson et al. (Int. J. Greenhouse Gas Control, 3, 11-19, 2009) have studied metal oxide sorbents such as manganese oxide (Mn2O3), copper oxide (CuO) and cobalt oxide (Co3O4) carried on a carrier substrate for supplying oxygen for the combustion of solid fuels. In their process, known as chemical looping combustion (CLC), fuel and oxidized metal oxide sorbents are placed in intimate contact with each other in a fuel combustion chamber. Oxygen is released from the metal oxide sorbent during combustion with the fuel thus reducing the oxidized metal oxide sorbent. The reduced metal oxide sorbent is then captured and recharged or regenerated with oxygen from air in an appropriate oxidation reactor under suitable conditions. The recharged oxidized metal oxide sorbent is then returned or “looped” to the combustion chamber for combustion with the fuel again.
A flue gas is produced during chemical looping combustion containing carbon dioxide, water vapor, a small amount of oxygen and other gaseous combustion products, depending on the stoichiometric proportions of the fuel and metal oxide sorbents. If the combustion occurs in the presence of a high percentage of oxygen, the combustion can occur at a relatively high temperature as compared to the same combustion process where air provides the oxygen for combustion. The relatively large quantity of inert nitrogen gas in air keeps the temperature in a combustion reactor at lower temperature as compared to oxyfiring with pure oxygen. In certain instances, this higher temperature may be advantageous. However, conventional combustion reactors, designed to operate at lower temperatures where air and fuel are combusted, can be damaged if they are operated at these higher temperatures. The present invention addresses the need to provide oxyfiring substantially free of nitrogen gas while allowing for the ready retrofit of conventional combustion reactors designed to operate at lower temperatures than would be provided by oxyfiring with a pure stream of oxygen.